pressure switch on off rapidly

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Old 10-05-11, 12:06 PM
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pressure switch on off rapidly

Hi there ladies and gents. I have a problem with a pressure switch not wanting to settle down. I realize there are many threads on here about this topic but all seem to involve a well pump or a holding tank.

I have this pump in a commercial building on the first floor of three stories. The city pressure comes in at about 45-50 psi which is ok for most operations. ideally we would like to keep it around 65-70 psi to handle surges from flush valves and autoclaves.

for a long time it had a furnas pressure switch (30-50 psi preset) running to a contactor powering a 5hp 3 phase pump and worked fine. The switch went bad and i replaced it with the same type switch. the pump would come on and turn off in rapid succession thus destroying shaft couplers every day. I swapped it for a switch with a 50-70 psi preset and even turned the off pressure up with hopes to limit the pressure to <80 psi.. Either the pump stays on too long or when it does go off, and tries to come back on, it starts jumping again.

the pump originally ran continuously and rarely, if ever, turned off. Pressure never exceeded 70 psi. Now if the motor runs continuously psi will exceed 80 which seems high.

i know the regulator could help to limit my pressure from the pump but i dont understand the short cycling. Any advice will be appreciated

here is a diagram i drew up for you to see my setup.

thanks thanks thanks

http://i1228.photobucket.com/albums/...ingdrawing.jpg
 

Last edited by chadwak; 10-05-11 at 02:11 PM.
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  #2  
Old 10-05-11, 01:41 PM
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We can't see that diagram without 'signing in'... if you want us to look at it, upload to a free account on Image hosting, free photo sharing & video sharing at Photobucket and give a link to your public album here.

From your description I don't understand why it ever worked fine...

I believe that you are going to need to install a PRESSURE TANK and treat the system downstream from the booster pump the same way that a well pump would be treated. I believe you need a TANK that will allow you to fill the tank, and then as the pressure tank empties from use, the pump kicks on again and refills it.

You may also find that this is a good application for a 'Cycle Stop Valve' ... someone will be along to explain that shortly, I'm sure!
 
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Old 10-05-11, 02:18 PM
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i put a link to photobucket a moment ago.

my theory is that the former switch was very far out of adjustment and never reached a cutoff point. As for the lower operating pressure, I surmise that the city pressure may have been slightly lower at one time (15-20 years ago) and the pump was never in danger of over pressuring the system.

http://i1228.photobucket.com/albums/...ingdrawing.jpg
 
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Old 10-05-11, 03:36 PM
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I do believe you will need a pressure tank on that system placed at the pressure switch location.

Is your pump controller an ON/OFF ? or is it a VFD ? (Variable Frequency Drive)

And possibly another check valve placed on the suction side of the pump to prevent backflow through the pump into the city supply.

When the pump raises the pressure and the switch cuts off, the flow will go right back through the pump and the pressure will drop to city pressure, and the switch will cut back on again, and off again...

With that huge of a pump (is the building really that large that you needed a 5HP pump?) you will need a fairly large pressure tank to suit the needs of the building and prevent the pump from cycling.

But, on the other hand, take a look at this website:

Cycle Stop Valves, Inc.

You would still need a tank, but not as large. I'm sure if you contact them they will be very helpful!
 
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